I just noticed that the developers of JabRef released version 4 in October.
JabRef is still our go-to recommendation when it comes to cross-platform reference management. The BibTeX support is great, and with BibTeX + MultiMarkdown you have an open source publishing toolchain at your fingertips. (On macOS, BibDesk is still our favorite, but JabRef is a close second even there.) Note that JabRef requires Java 1.8+ to run.
The integration with paper recommendation service “Mr.DLib” sounds interesting. I have no clue if this is any good in practice; since I don’t do any research at all at the moment, I cannot evaluate that feature. You’re welcome to share your experience with the rest of us in the forum comments!